Almost all of the network drivers in this patch have been optimized for performance with respect to PHY probing.
Prior to this patch, network drivers would periodically (e.g. every two or three seconds) communicate via the MII to the PHY chip connected to the NIC, to determine the speed and duplex of the current media connection.
The problem is that, while the PHY is being probed, packet loss can occur. The drivers in this patch contain an optimization to not probe the PHY, as long as there have recently been some packets received. This gives maximum performance for most users.
However, there is a nasty scenario: the NIC is connected to a 100 Mbit full-duplex link. The cable is rapidly unplugged and immediately replugged into a 10 Mbit half-duplex hub, which also has a steady stream of (e.g. broadcasted) received packets. In this scenario, because of the steady stream of received packets, the network driver won't probe the PHY, and will still think it's in 100 Mbit full-duplex. This is a problem, because the NIC isn't listening before it transmits; it's still full-duplex, on a half-duplex link. Excessive collisions and out-of-window collisions will result in packet loss.
However, if you leave the cable unplugged for three seconds before plugging it into another hub, the driver probes the PHY and relearns the media parameters and reprograms the NIC with the appropriate duplex.
If you need to rapidly unplug and replug the cable into network boxes with different duplexes, you should specify probe_phy=1 to the network driver, to force it to always periodically probe the PHY. Packet loss may result during this probing, but you will know that the driver is always in sync with the PHY with respect to the media.
For maximum performance, the default is probe_phy=0.